Reinforced concrete detailing: the cover to the main reinforcement may be reduced by up to one-third of the specified cover but not less than which absolute minimum?

Introduction / Context:
Concrete cover protects reinforcement against corrosion and fire and ensures adequate bond. Tolerances are allowed during construction, but codes limit how much the actual cover can deviate below the specified value. This question targets the commonly cited absolute minimum reduction permitted when a fractional reduction (one-third) would otherwise reduce cover too much.

Given Data / Assumptions:

• Specified nominal cover is determined by exposure, bar diameter, and fire rating.
• Construction tolerances permit reducing cover up to one-third.
• An absolute floor (minimum) applies: the reduced cover must not be less than a fixed millimetre value.

Concept / Approach:
While one-third reduction accounts for practical placement variability, a hard minimum ensures durability is not compromised for small nominal covers. Many exam-oriented references and detailing handbooks state that even after reduction, cover should not fall below 5 mm for typical reinforced members, balancing constructability with durability needs in benign exposures. Designers must still check exposure-specific minimum covers: severe or marine environments may require larger minimums irrespective of tolerance rules.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

Common Pitfalls:

Final Answer: